The present invention relates to a bead core for a pneumatic vehicle tire, with the bead core comprising a plurality of spirally wound layers, each of which in turn comprises load-carriers that are disposed next to one another. The present invention also relates to a method for producing such bead cores, and to a winding reel as an apparatus for carrying out such a method.
Furthermore, the present invention relates to a bead core for a pneumatic vehicle tire that comprises load-carriers in the form of a wire that are arranged in a plurality of layers, whereby the thickness of the layers corresponds essentially to the thickness of the wire of the load-carriers and the width of the corresponds essentially to n times the thickness of the wire of the load-carriers. The number n corresponds to the number of load-carriers within each layer, with the load carriers being disposed adjacent to one another within each layer. The layers are wound from a single ply of adjacently disposed load-carriers. The width of the ply varies at most only at the beginning and/or at the end of the ply, whereby the beginning of the ply and/or the end of the ply have at most a length that corresponds to the circumference of the bead core.
In addition to so-called cable cores where the load-carriers are twisted relative to one another in conformity with various patterns, the subject wound cores, so-called Pierce cores, have gained extensive ground due to the fact that they are quite economical and due to the great freedom with regard to the configuration of the core cross-section. To produce this type of core, an endless layer is first formed from load-carriers that are disposed next to one another and are embedded in rubber. Aside from the other dimensions, in particular the considerably more compact positioning of the load-carriers or wires, such a layer in principle resembles a flat or ribbon cable that is known from the electrical field, where the insulation would be analogous to the rubber. The number of load-carriers of a layer that are disposed next to one another depends upon the size of the tire and is between three and nine. From such a layer, the Pierce core is built up by attaching the end of the layer to the surface of a cylindrical reel, and then winding the layer to form several windings, whereupon the layer is cut and the end of the layer is pressed on. Depending upon the size of the tire, three to eight windings are applied. Analogous to the language utilized when describing carcasses, the individual windings of a bead core that is produced in such a way are generally designated as "layers", and this language will also be utilized in the subject application. Bead cores produced in the manner described are designated in the art as Pierce cores.
In the following, the individual windings of a bead core produced according to the above described process will be designated by the term "layer" in correspondence to their appearance in a cross-sectional view. With Pierce cores, the core layers extend generally parallel to the seating surface of the rim, i.e., essentially axially. The wires within one layer are arranged adjacent to one another. The layers are wound from a single ply. The layers have a thickness that corresponds to the sum of the thickness of the wire and of the rubber material in which the wire is embedded. The thickness of the rubber material is smaller by at least a factor of 10 when compared to the thickness of the wire. Accordingly, in the following it is assumed that the thickness of the layer corresponds essentially to the thickness of the wire.
Aside from the aforementioned advantages, the Pierce cores have the drawback that the inner radius of the bead core unevenly varies at the beginning of the first winding by the thickness of one layer. This uneven fluctuation of the radius continues to a certain extent into the carcass and leads to a systematic inaccuracy of the trueness or concentricity. Although this inaccuracy can to a large extent be kept to a minimum by wrapping the layers onto the tire build-up machine in a particularly skillful manner, this of course makes it more difficult to automate the process of constructing the tire carcass. Although this source of inaccuracies is lessened if the thickness of the layers is reduced and thereby the number of layers is increased, the construction of the bead core then becomes more expensive and ultimately, if this direction of development is followed to its extreme, becomes susceptible to warping.
A bead core is known from DE-OS 28 20 191 which describes the use of a metal band (in the same layer arrangement that corresponds to the Pierce core) instead of a ply formed from wires. The metal band has a greater bending resistance with respect to axial warping due to its alignment in the axial direction. However, these bead cores have not been successful due to their differing behavior in various load situations.
A further development of the Pierce core is known from DE-OS 37 38 446 in which, in order to achieve a more uniform structure of the ply, the beginning and the end of the ply are cut in a slanted fashion relative to the axial direction. Accordingly, not all of the wires of the first ply will end at the same circumferential location. The desired effect, however, may only be observed when the slant is quite great, for example, when the cutting location is distributed over at least one quarter of the circumference. The individual wire cross-section may not be cut at such a sharp angle, however, because the wire would than act lake a knife thus damaging the second ply as well as the bead core sheathing. The cutting should therefore be carried out (according to claim 2 of the cited reference) in a stepped manner which, on the other hand, will increase the production coasts.
Two more methods for making the beginning of the ply more uniform are disclosed in EP-0 303 514 A1. These methods comprise: a slanted cutting (when viewed from the side), i.e., at an angle relative to the radial direction, and the incorporation of the beginning of the wire (after being cut in the axial direction) between different layers. A sufficiently slanted cut relative to the radial direction does not result in the same amount of damage to the surrounding components as described in the previous prior art reference, but is somewhat dubious when lateral forces must be transmitted. Furthermore, the required cuts at such a great angle are hardly performable due to the ductility of the wire. The incorporation of the beginnings of the wire between different layers in time-consuming and requires that the production be interrupted for each cutting step.
From U.S. Pat. No. 3,170,662 as well as from DE-OS 24 09 816 and U.S. Pat. No. 3,942,574 a bead core embodiment is known which is formed from a single wire that is wound into a plurality of separate spirals having an essentially radial cross-section. The individual spirals are then attached to one another in their axial direction, whereby the fastening is solely achieved via the frictional connection between the rubber material in which the wire is embedded. These bead cores also have the beginning of the wire located at the radially inner side of the individual spiral and are thus suffering from the same disadvantages as the Pierce core. Furthermore, the individual spiral windings are only connected via the frictional force between the rubber material.
From claim 6 of DE-OS 37 38 446 a further development of a bead core manufactured in the aforementioned manner from individual disk-like elements is known, whereby the respective beginnings of the wire of the individual layers (disposed at the radially inner side) are spaced from one another over the inner circumference so that these discontinuities are evenly distributed. The handling and logistics during manufacture of such a bead core are difficult. Furthermore, each one of the individual beginnings of the wire still represents about half of the discontinuity that results from arranging all of the beginnings of the wire at the same circumferential location. However, since more than twice the number of discontinuities result, the noise level resulting from this arrangement is increased but shifted to a higher frequency which may be dampened more easily.
U.S. Pat. No. 1,503,985 refers to a bead core made from sheet metal that is wound into a helix. The width of the sheet metal continuously increases from a minimum to a maximum so that the resulting shape has a triangular cross-section.
It is an object of the present invention to provide a bead core that while in principle maintaining the particularly economical winding-up of a single rubber-coated load-carrier layer, and without well-aimed manual adaptation of the length of the amount that the carcass plies are looped about the bead core, makes it possible to provide a pneumatic vehicle tire that has an improved concentricity.